1. Field of the Invention
This invention relates to the preparation of formylphosphonic acid, its salts, and its esters, and particularly to novel processes for the preparation of formylphosphonic acid by the catalytic decomposition of a (phosphonomethyl)amine N-oxide compound in the presence of a decomposition catalyst.
2. Description of Related Art
Phosphorus-containing compounds such as formylphosphonic acid are important precursors for the synthesis of organophosphorus compounds. Such organophosphorus compounds have numerous applications. For example, formylphosphonic acid can be used as a precursor in the synthesis of N-(phosphonomethyl)glycine, a highly effective commercial herbicide (common name glyphosate, available under the trade name Roundup.RTM.) useful for the control of a large variety of weeds. Formylphosphonic acid can alternatively be used as an advanced intermediate in the preparation of medicinally important compounds such as the antiviral agent phosphono hydroxyacetic acid. As a reagent or intermediate, formylphosphonic acid has potential for chemical transformation at the carbonyl, phosphorus, or hydroxyl moieties.
Researchers have reported electrochemical processes in which formylphosphonic acid forms. For example, Wagenknecht (Synth. React. Inorg. Met.-Org. 4:567-572 (1974)) spectrophotometrically observes or isolates formylphosphonic acid in the electrochemical oxidation of nitrilotris(methylenephosphonic acid) to the secondary amine. A similar reaction is reported in U.S. Pat. No. 3,907,652. In J. Electrochem. Soc. 123:620-624 (1976) Wagenknecht reports the electrochemical oxidation of substituted iminodimethylenediphosphonic acids to produce the secondary amine. In that study, formylphosphonic acid was isolated in unreported yield as a side product. Wagenknecht, et al. again reports the formation of formylphosphonic acid as a side product in the electrochemical oxidation of nitrilotris(methylenephosphonic acid) in Synth. React. Inorg. Met.-Org. 12:1-9 (1982). However, these reactions suffer from several shortcomings. Yields of formylphosphonic acid are poor or unreported. Wagenknecht (1982) reports that formylphosphonic acid degrades under the electrolysis reaction conditions. The electrochemical reaction requires the addition of a strong hydrochloric acid solution which presents safety, environmental, and equipment corrosion problems. Electrochemical methods generally require an external power source and other equipment which typically have higher maintenance needs and costs than do non-electrochemical reactions. It would be advantageous to have a method for the preparation of formylphosphonic acid in high yield which does not require specialized electrochemical equipment and does not require the handling of large quantities of strong mineral acids.
Hershman et al., in U.S. Pat. No. 4,072,706, disclose a process in which (phosphonomethyl)amines are oxidized with molecular oxygen in the presence of an activated carbon catalyst to cleave a phosphonomethyl group and produce a secondary amine. According to Hershman et al., formylphosphonic is produced as an intermediate cleavage fragment, with the fragment undergoing hydrolysis in a second step to formic acid and phosphonic acid. Hershman et al., however, identify formylphosphonic acid as an intermediate cleavage fragment in only one reaction run and although the yield is unreported it is apparently low. In addition, Hershman et al. fail to disclose any means to limit the hydrolysis of the intermediate cleavage fragment.
Disclosures have been made of a process wherein N-(phosphonomethyl)iminodiacetic acid N-oxide is catalytically decarboxymethylated to form N-(phosphonomethyl)glycine, carbon dioxide, and formaldehyde. For example, Fields, et al. disclose such a reaction in U.S. Pat. No. 5,043,475. However, the Fields, et al. decarboxymethylation is highly selective for an acetic acid arm of the N-oxide and cleavage of the phosphonomethyl arm is not reported.
Thus, a need exists for a convenient, environmentally-compatible, safe, and cost-effective process for the reaction of aminomethylphosphonic acid derivatives to produce formylphosphonic acid in high yield with minimal degradation.